An anisotropic conductive film (ACF) is formed by dispersing conductive particles in an insulating binder resin that functions as an adhesive agent. A conventional anisotropic conductive film is in the form of a sheet, as a binder resin composition having conductive particles dispersed therein is applied onto a base film. When an anisotropic conductive film is used, the anisotropic conductive film is inserted between the bumps of an electronic component and the electrode terminals of a wiring board, and heat and pressure are applied to the anisotropic conductive film with a heating/pressing head. As a result, the conductive particles are flattened by the bumps and the electrode terminals, and the binder resin is hardened in such a situation. Thus, an electrical and mechanical connection is achieved. At the portions without bumps, the conductive particles remain dispersed in the binder resin, and continue to be electrically insulated. Accordingly, electrical conduction is provided only at the portions with the bumps. The thickness of the anisotropic conductive film is equal to or greater than the height of the bumps of the electronic component and the electrodes of the wiring board, and extra adhesive components are pushed away to the peripheries of the electrodes by the pressing force from the heating/pressing head.
In many anisotropic conductive films, the proportion of conductive particles is 5 to 15 volume % relative to the volume of the adhesive components. If the proportion of conductive particles is less than 5 volume %, the amount of conductive particles existing between bumps and electrode terminals (this amount is generally called “particle trapping rate”) becomes smaller, and conduction reliability might decrease. If the proportion of conductive particles exceeds 15 volume %, on the other hand, joined conductive particles exist between adjacent electrode terminals, and might cause short-circuiting.
However, if the proportion of conductive particles dispersed in an anisotropic conductive film is simply optimized, most of the conductive particles is pushed away at the time of pressure bonding, and many conductive particles fail to contribute to conduction. Also, the conductive particles pushed away form particle pools between adjacent electrode terminals, and are in danger of causing short-circuiting. As the interval between the electrode terminals is made shorter, the possibility of short-circuiting becomes higher, and it is not possible to cope with high-density packaging.
In view of such circumstances, there have been suggested techniques by which conductive particles in an anisotropic conductive film are not randomly dispersed, but are uniformly dispersed in a binder resin layer (see Patent Literatures 1 and 2).